Insulated wires which are covered with electrical insulators are used in large quantities for coils integrated in various electrical apparatuses. In recent years, the step of working and winding coils of insulated wires is increased in speed and become rationalized, and the coil winding operation is changing from a manual winding operation to one conducted by automatic coil winding machines. The insertion of coils into stator slots is also automated.
In winding a coil automatically, however, since a great tension is applied to the insulated wire, the stress on the coating film is great and hence the insulated wire is inevitably susceptible to damage. Further, since the insertion of a coil into a stator slot is carried out by machine instead of manual operation, the wire undergoes a greater pressure. Under these circumstances, rubbing between the electric wires, or between the electric wire and an object that comes in contact with the electric wire, easily occurs, and defective insulation of the coil is apt to result.
Further, by making the space factor of the insulated wire in the stator slot where the coil is placed as large as possible, the apparatus can be made small in size and can be lowered in cost. Therefore it is desired to reduce the outer diameter of the electric wire. In recent years, with the reduction in diameter, it is demanded that the diameter of the conductor be kept intact or be increased, in order to increase the power of apparatuses, and therefore it is needed to decrease the build of the insulating coating.
When winding of coils and insertion of coils into stator slots are automated, however, making the insulating coating thin increases the frequency of damage to the coating, resulting in an increase in the rate of the occurrence of defective insulation of the coils.
To solve the above problems, a decrease in the friction coefficient between electric wires, or between an electric wire and an object that comes in contact with an electric wire (e.g. between a metal rod and layer insulating paper), and improvement in the strength of the coating are considered. The lower the friction coefficient is, the easier the winding of a coil is, and the higher the strength of a coating is, the more decreased the damage to a coil is when the coil is wound and the coil is inserted into a stator slot (hereinafter, these operations are referred to as the working of a coil).
As conventionally practiced means of lowering the friction coefficient, there is a method in which a lubricant is applied to the surface of an electric wire, and a method in which a lubricant is added to an electrical insulating coating material and the coating material is applied and baked.
As means of improving the strength of a coating, an insulated wire having a polyamide-imide coating material applied and baked is generally used. Since, in comparison with insulated wires having other resin (e.g. polyester, polyurethane, polyesterimide, polyesteramide-imide, or polyimide) applied and baked, the insulated wire having a polyamide-imide resin coating material applied and baked is high in mechanical strength and excellent in abrasion resistance, it is frequently used when the conditions of the working of a coil are severe.
Recently, however, the conditions of the working of a coil have become increasingly severe, and, in many cases, even when the friction coefficient is decreased as described above or an electric wire having a polyamide-imide resin coating material applied and baked is used, damage to the coating cannot be prevented.
Thus, as a means of preventing the coating from being damaged at the time of the working of a coil, a method for improving the adhesion between an insulating layer and a conductor is suggested. As specific examples of an insulating coating material that achieves this desired adhesion, there are suggested: 1) a heat resistant coating material made from a polyamide-imide resin, an alkoxy-modified amino resin, and benzotriazole (JP-A-3-37283 ("JP-A" means unexamined published Japanese patent application), and 2) a coating material made from a polyamide-imide resin and a trialkylamine (JP-A-6-111632).
In the wires obtained by using these means, although an effect can be observed in the repeated scrape test (a test wherein a relatively small load is applied to the wire and the coating is abraded with a beads-needle), no effect is recognized in the unidirectional scrape test (a test stipulated in JIS C 3003, wherein, while a load is applied increasingly on the wire, the coating is scratched with a piano wire). Recently, the latter test is regarded as important as a coating damage test.
In the case of a wire intended to be improved only in adhesion, as the build of coating is made thin, the repeated abrasion (scraped) value is lowered, in many cases approximately to the same level as that of the conventional wire that is not intended to be improved in adhesion.
On the other hand, a method is suggested in which many rigid structures are introduced in the molecules of a resin, to improve the strength of a coating, to decrease scars made by coil working, and JP-A-6-196025 describes an insulated wire having an insulating coating with the tensile strength and the tensional modulus defined. In such a wire, a remarkable effect is observed in the unidirectional scrape test, and even when the coating is made thin, damage to the coating at the time of the working of a coil can be prevented. However, in comparison with conventional wires, in such a wire, the level of the flexibility after the stretching, and the level of the flexibility after the heat history, are low, and particularly when such a wire is subjected to severe bending, the flexibility is not satisfactory, and therefore there is a risk that the coating may cracked or broken.
As an example of a wire having multiple layers in which different coatings are layered one on the other, to improve the mechanical strength and the heat shock resistance of the final coating, there is a technique described in JP-B-62-21203 ("JP-B" means examined Japanese patent publication). This example discloses an insulated wire that is provided with a polyamide-imide resin as an undermost layer, an aromatic polyimide resin as an intermediate layer, and a polyamide-imide resin as an overlayer, to improve the mechanical properties of the coating and the heat shock resistance after the impregnation treatment with a varnish. In this wire, the repeated abrasion value is found to be improved, and when the wire is heat-treated after the impregnation treatment with a varnish, the varnish pulls the coating, and this force is mitigated in the intermediate layer, to prevent the coating from being broken or cracked. However, although the repeated abrasion value is improved, the wire has no effect on the unidirectional abrasion. Further, when the coating of the wire is made thin, the repeated abrasion value is lowered, and therefore further improvement is required to prevent the coating from being damaged at the time of the working of a coil.